Raid boss

A raid boss is a specialized boss which is designed to be fought by multiple players, typically of significantly higher difficulty than normal bosses and requiring specialized strategy and/or loadouts. Within Borderlands and Borderlands 2, raid bosses have the title "the Invincible" and are optional encounters, and need not be defeated or even fought to complete the main story. (Some of them must be defeated to complete the achievements/trophies for their respective content, however.)

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Borderlands

Originally, Borderlands lacked anything which could be considered a true raid boss. All bosses, even optional encounters, were designed such that single players could defeat them. The first raid boss to appear in the Borderlands series was added by a DLC, The Secret Armory of General Knoxx. The optional mission You. Will. Die. tasked players with defeating the appropriately named Crawmerax the Invincible. Though not specifically designed to be undefeatable by a single player, Crawmerax's high difficulty coupled with hard-to-reach vulnerable spots—especially the one located on his back—made him effectively off-limits to most solo players.

Borderlands 2

Vermivorous the Invincible, another raid boss in Borderlands 2

On its release, Borderlands 2 was advertised specifically as having a specially designed raid boss meant to be fought by four level 50 characters. The boss, Terramorphous the Invincible, was inspired by Crawmerax, and meant to be more difficult than him. Much of the design of Terramorphous references Crawmerax, including the arena used for the fight as well as the mission name: You. Will. Die. (Seriously.)

A second, somewhat hidden raid boss was also included. Vermivorous the Invincible is the most advanced metamorphosis that a varkid can reach. Though it can theoretically be encountered at any level and with any number of players on True or Ultimate Vault Hunter Mode (it cannot spawn on Normal Mode), the chance of a varkid progressing through the necessary metamorphoses is highest with four level 72 characters. Vermivorous also has health far in excess of Terramorphous, estimated at around 200% of Terramorphous's health.

Borderlands 2 DLC

The DLCs for Borderlands 2 have also included a number of raid bosses. To date, all released DLCs have included at least one raid boss. Accessing the DLC raid bosses requires a payment of eight Eridium, except for the Ancient Dragons of Destruction who require 20, and Dexiduous the Invincible who requires between 95 and 99.

Mr. Torgue's Campaign of Carnage

The second DLC, Mr. Torgue's Campaign of Carnage, adds a third raid boss, Pete the Invincible. Pete is an upgraded version of an earlier standard boss, Pyro Pete, who has been enhanced with both fire and corrosive attacks, as well as extremely high health. Like the previous two DLC raid bosses, Pete is a Seraph Guardian and will drop Seraph Crystals.

Borderlands: The Pre-Sequel

The raid boss in Borderlands: The Pre-Sequel is The Invincible Sentinel (and, by proxy, its second form The Invincible Empyrean Sentinel). The Invincible Sentinel is effectively a stronger, upgraded version of the campaign final boss The Sentinel. The boss is encountered in Eleseer, by placing moonstone in each of the four altars in front of the vault entrance before walking through it. In order to fight the Invincible Sentinel, the quest The Bestest Story Ever Told must be accepted or already completed.